The invention relates to a yarn processing system including a weft yarn feeding device and a weaving machine, and to a method for feeding weft yarn with a feeding device into a weaving machine.
In a gripper weaving machine, the yarn tension profile follows a curve similar to a kardioide caused by accelerations and decelerations of the yarn. In the insertion starting phase, a predetermined yarn tension level is adjusted by a controlled yarn brake such that the bringer gripper properly takes the weft yarn. During the further stroke of the bringer gripper, the braking effect ought to be reduced as far as possible since the yarn tension then increases due :the ion. Shortly prior to the transition phase from the bringer gripper to the taker gripper, the braking effect again has to be increased for a proper transition. During the subsequent acceleration of the taker gripper, the braking effect again ought to be reduced as far as possible, until it finally is to be increased shortly before the insertion end to position the weft yarn in a stretched manner within the shed. In a projectile weaving machine, the braking effect of the controlled yarn brake both is increased in the insertion starting phase and shortly before the insertion end, but is to be reduced as far as possible in the intermediate phase. In a jet weaving machine, an increased braking effect of the yarn brake is of advantage at the insertion end phase to dampen the effect of a dangerous whiplash.
In a yarn processing system known from EP 03 57 975 A, the braking effect is varied during an insertion cycle by means of a signal representing the yarn tension and by a closed regulation loop including the yarn brake. Different positions of the yarn tip within the shed are allocated to different yarn tension values to adapt the overall yarn tension profile to the weaving conditions. Since the yarn tension profile varies among insertion cycles with the same yarn material due to the weft yarn withdrawal action of the weaving machine and also depends upon other influences, it has proved of advantage not to operate with a strictly fixed program of the braking effect of the yarn brake but to control the yarn brake in view of the actual yarn tension profile. The tensiometer, the probe of which is in direct contact with the weft yarn, generates signals representing the actual yarn tension or a signal train representing the actual yarn tension profile, respectively. With the help of this actual signal, the signal evaluation assembly regulates via the yarn brake a nominal yarn tension profile. For the regulating operation, the regulating loop of the tensiometer, the signal evaluation assembly and the yarn brake are closed by the yarn itself. The permanent mechanical contact between the weft yarn and the probe of the tensiometer, however, causes a permanent deterioration of the regulating accuracy. The problem results from a stretching tension caused by the probe within the weft yarn between the feeding device and the insertion device of the weaving machine. Said stretching tension is active even in pauses between insertion cycles, and influences the tensiometer to generate a signal with a value which drifts due to varying exterior circumstances. In addition, operation dependent vibrations in the yarn processing system also influence the stretching of the weft yarn and will be detected by the tensiometer. Even with the weft yarn at rest the signal shows a considerable drifting tendency. As a result, the signal evaluation assembly never has a predetermined or fixed signal value as a proper reference for the regulation. A reliable calibration or resetting, preferably to zero, cannot be carried out as it is needed for high regulation accuracy.
U.S. Pat. No. 5,476,122 A discloses a yarn processing system where a lamella yarn brake is regulated in a closed regulation loop with signals emitted by a tensiometer. FIG. 2 of said publication illustrates that strong stretching tension is active in the weft yarn in pauses between insertion cycles and illustrates that the stretching tension may have differing levels in different pauses (drifting). Since there is no point in time without any detected falsifying yarn tension, no reliable calibration or resetting can be carried out for the tensiometer.
It is an object of the invention to create in a structurally simple way a yarn processing system having high regulation accuracy of the yarn brake control, and to provide a method for improving the regulation accuracy for the yarn tension in a yarn processing system.
Said object is achieved according to the invention by providing a separation device close to the tensiometer within the yarn path to temporarily separate the probe and the weft yarn to allow the signal evaluation assembly to generate a zero yarn tension signal. The object is also achieved by carrying out the method discussed below.
Independent from the magnitude of the stretching tension within the weft yarn which is active even in pauses between insertion cycles, a temporary separation of the weft yarn from the probe of the tensiometer is carried out in the yarn processing system. A stable reference value is gained in the signal evaluation assembly for calibrating or resetting when the weft yarn is separated from the probe. Said reference value is a clear zero yarn signal because the tensiometer does not generate any signal or always the same basis signal when the weft yarn is temporarily separated from the probe. With the help of said zero yarn tension signal, calibrating or resetting can be carried out in a reliable and simple way.
According to the method the zero yarn tension signal intentionally is generated for the purpose of calibration of the signal evaluation assembly. The regulation accuracy is significantly increased by rerlatively simple structural methods. The separation device stays passive during the normal regulation process and does not then have any influence the weft yarn tension control.
The separation device is actuated by an adjustment drive which expediently is connected to a control device. An information signal is transmitted via the information connection to the signal evaluation assembly, such that the signal evaluation assembly can use the zero yarn tension signal as a reference for calibrating or resetting, respectively.
The separation device includes at least one lifting element. The weft yarn is lifted from the probe of the tensiometer during the separation in a structurally simple way. The lifting element, when carrying out the separation, only engages at the weft yarn for a calibration step of the evaluation assembly. This is done while the weft yarn has stopped. Said engagement thus does not influence the subsequent yarn run.
Alternatively, the probe of the tensiometer or the tensiometer itself, is adjusted in relation to the weft yarn into the passive position to separate the probe from the weft yarn.
In one embodiment, a rod-like or yarn eyelet-like lifting element is provided as a yarn deflector which is structurally simple.
In one embodiment, the adjustment drive includes an electromagnet, an electric motor or a pneumatic cylinder, and operates in a structurally simple and reliable way.
The signal evaluation assembly includes a calibration part for calibrating or resetting. Said part responds to the zero yarn tension signal as soon as the separation has taken place.
The invention also relates to a method for intermittently feeding weft yarn with a feeding device into a weaving machine, including temporarily separating the weft yarn and tensiometer probe, deriving a zero yarn tension signal during the separated state, calibrating the yarn tension signal on the basis of the zero signal, and registering the calibration as a basis for an upcoming yarn brake regulation cycle.
In one embodiment, the calibration is made as soon as the weft yarn has stopped and/or is not braked anywhere else.
According to the invention the weft yarn and the scanning probe of the tensiometer are temporarily separated from one another to gain a valuable and clear zero yarn tension signal for a calibration step. This is of particular importance for tensiometers operating with a piezo transducer. Alternatively, the separation is also useful for tensiometers employing strain gauge strips, a capacity sensor, a piezo resistive element, or for other electronic yarn tension measuring principles like triboelectrical devices, each of which needs contact of the yarn for sensing the yarn tension.